The present invention relates to pulse height analysis in a multi-channel analyzer, and more particularly to the accurate detection and height analysis of pulses generated by a radiation detector in a multi-channel analyzer. Even more particularly, the present invention relates to accurate real-time detection and height analysis using digital signal processing of pulses generated by a scintillation-type radiation detector in a multi-channel analyzer.
The detection of sub-atomic particles using ionization chambers, proportional counters, Geiger counters, scintillation counters, Cadmium Telluride detectors, and semiconductor detectors, is known in the art of radiation/particle detection. The scintillation counter is preferred for use in detecting particles that arrive as few as only a few nanoseconds apart. The scintillation counter is also useful in determining the energy of detected sub-atomic particles, because the amplitude of the output pulse generated by the scintillation counter is proportional to the energy of the incident particle.
One way in which scintillation counters are utilized is in what are known as voltage window analyzers. Voltage window analyzers generate an output pulse (which is generally a digital pulse) in the event their input is an electrical pulse having a height (i.e., peak voltage amplitude) that exceeds a lower voltage threshold but that does not exceed an upper voltage threshold. The range of voltages between the lower voltage threshold and the upper voltage threshold is referred to as the voltage "window" or channel.
Referring to FIG. 1, a graph is shown of an sample input signal to and a sample output signal from a voltage window analyzer. Time is shown on the horizontal (abscissa) axes while voltage is shown on the vertical (ordinate) axis. A first (in time sequence) input pulse 10 is shown having a peak voltage amplitude that is less than the lower voltage threshold 12. In response to the first input pulse, the output 14 of the voltage window analyzer remains "not asserted," thereby indicating that the first input pulse 10 does not have a peak voltage amplitude that falls within the "window" 16.
A second input pulse 18 is shown having a peak voltage amplitude that falls between the upper voltage threshold 20 and the lower voltage threshold 12. As a result, the voltage window analyzer generates an output pulse 22, i.e., becomes "asserted" for a prescribed period of time T, in response to the second input pulse 18, thereby indicating that the second input pulse 18 has a peak voltage amplitude that falls within the "window" 16 or channel.
A third input pulse 24 is shown having a peak voltage amplitude that falls above the upper voltage threshold 20. As a result, the voltage window analyzer output 26, remains "not asserted" in response to the third input pulse 24, thereby indicating that the third input pulse 24 has a peak voltage amplitude that does not fall within the "window" 16.
In this way, voltage window analyzers are able to detect input pulses generated by a scintillation-type (or other-type) detector that correspond to incoming particles of energies that are within the "window" 16. Note that the "window," while represented as a window of voltages, corresponds to a window of particle energies.
In practice, such voltage window analyzers have several "windows," or channels, that are adjacent to one another along a voltage scale 28, i.e., vertical axis. Typical voltage window analyzers are capable of detecting the voltage amplitudes of input pulses with a resolution that corresponds to the height of each voltage window. In a typical voltage window analyzer, several hundred, or even more than one-thousand, "windows," or channels, are utilized. Such voltage window analyzers are typically implemented using a sample-and-hold circuit in combination with an analog-to-digital converter.
Problematically, voltage window analyzers, or multi-channel analyzers (MCAs) as they are commonly called, suffer from inaccuracies caused by several phenomenon. First, the baseline of the output signal from the scintillation counter may shift (or drift) over time due to interference from high frequency communications lines, solenoid activations, motor activations etc. that affect the power supply to the multi-channel analyzer. A fourth input pulse 30 is shown in FIG. 1 that is an example of a scintillation-type detector's output signal (or detector signal) that has been baseline shifted, i.e., that has been affected by baseline shift (or drift). As shown, the fourth input pulse 30 has a peak (measured from the baseline 32 of the fourth input pulse 30 to the peak 34 of the fourth input pulse 30) that falls within the "window" 16. Problematically, however, the peak 34 of the fourth input pulse 30 is not detected as falling within the "window" 16 because the amplitude of the peak 34 (measured from the horizontal axis 36 to the peak 34 of the fourth input pulse 30) has been increased by the baseline shift (measured from the horizontal axis 36 to the baseline 32 of the fourth input pulse 30) to a voltage above the upper voltage threshold 20. In the case of a multi-channel analyzer, the peak 34 of the fourth input pulse 30 will be detected as falling within a "window" or channel that is above the "window" 16 shown in FIG. 1. Thus, because such baseline shift results in the detection of particles of higher energies than those that are actually impinging upon the scintillation counter, a multi-channel analyzer that is able to accurately detect and compensate for baseline shift is highly desirable.
Referring to FIG. 2, the problem of noise that is present on the detector signal is illustrated. Noise is introduced on the detector signal by high frequency equipment, motors, compressors, solenoids, pumps etc. that may or may not share a common electrical energy source with the multi-channel analyzer. Problematically, such noise creates uncertainty in the location of the peak of an electrical pulse within the electrical signal. Similarly, noise causes uncertainty in the baseline of such electrical signal.
As illustrated in FIG. 2, wherein amplitude is represented on the horizontal axis and time is represented on the vertical axis, the electrical pulse reaches its peak during time T.sub.1. Unfortunately, the exact time and amplitude at which the electrical pulse reaches its peak cannot be determined due to the noise present in the detector signal. Instead, the noise results in the generation of several apparent or false peaks 40, 40' in the detector signal rather than one distinct peak.
After peaking sometime during time T.sub.1, the electrical pulse subsides during time T.sub.2, and reaches its baseline sometime during time T.sub.3. Unfortunately however, the exact time and the amplitude at which the electrical pulse reaches its baseline cannot be accurately detected due to the noise. Thus, a great deal of uncertainty in both the location of the peak and the baseline of the electrical signal, and therefore the pulse height, is caused by the noise in the electrical (or detector) signal. Because the accurate determination of pulse height is dependent on the accurate location of the pulse's peak and baseline, a multi-channel analyzer that can accurately detect pulse height in the presence of noise on the electrical pulse is desirable.
Referring to FIG. 3, wherein amplitude is represented on the vertical axis and time is represented on the horizontal axis, the problem of pulse pileup is illustrated. Pulse pileup occurs when more than one electrical pulse is generated by the scintillation counter in response to several particles impacting the scintillation counter within a very short period of time. As a result of pulse pileup, a first pulse 50 generated by the scintillation counter in response to a first particle does not reach its baseline by the time a second pulse 52 is generated by the scintillation counter in response to a second particle. As a result, the amplitude of the peak 54 generated in response to the first and second particles is the sum (i.e., a "sum" pulse) of the amplitudes of the first and second pulses 52,50. In response to such sum pulses, multi-channel analyzers that are unable to detect pulse pileup will generate an output signal indicating the detection of a single pulse having a peak amplitude within a channel (or "window") that corresponds to a range of energies that are greater than the range of energies, i.e., channel 58, in which either the first or second pulses 52,50 peak. Unfortunately, such misdetection of higher energy particles creates inaccuracy and uncertainty in the pulse height analysis performed by some multi-channel analyzers. Therefore, a multi-channel analyzer that can accurately detect and account for pulse pileup is highly advantageous.